1. Field of the Invention
The present invention relates to a spark plug in which a metallic shell is crimped for integral fixing of the metallic shell and an insulator.
2. Description of the Related Art
Conventionally, an internal combustion engine uses a spark plug for ignition. An ordinary spark plug includes a metallic shell which holds an insulator into which a center electrode is inserted, and a ground electrode welded to a front end portion of the metallic shell. Furthermore, the other end portion of the ground electrode and a front end portion of the center electrode face each other to thereby form a spark discharge gap. Thus, spark discharge is generated between the center electrode and the ground electrode.
In such a spark plug, a crimp portion provided at the rear end of the metallic shell is crimped with a step portion formed on an outer circumferential surface of the insulator held by means of a step portion formed on an inner circumferential surface of the metallic shell, whereby the insulator and the metallic shell are integrally fixed to one another. Talc and a packing are accommodated in the interior of the crimp portion. In order to reliably fix the metallic shell and the insulator together, an end of the crimp portion is brought into contact with the insulator.
In recent years, implementation of improved output and low fuel consumption have been required of automobile engines. Under such circumstances, in order to ensure the requisite degree of freedom in designing engines, there has been a need for a reduction in spark plug diameter and an increase in reach. As a result, the outer diameter of a trunk portion of the insulators has been reduced, with a resultant decrease in strength.
In the case where such a small-diameter spark plug is configured such that the crimp portion is in direct contact with the insulator, if the insulator is subjected to an external impact; for example, as a result of a spark plug mounting tool, such as a wrench, striking the insulator when the spark plug is being mounted to an engine, the insulator may be damaged or fractured at the end of the crimp portion in contact with the trunk portion. In order to prevent such fracture of the insulator, the insulator and the metallic shell are desirably fixed together so that the end of the crimp portion of the metallic shell does not come into contact with the trunk portion of the insulator (refer to, for example, Patent Document 1).
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. H10-125444.